Referring to FIG. 1, photovoltaic (PV) cells 100 are semiconductor devices that convert light into electricity. A plurality of PV cells 100 may be connected electrically in series and/or parallel circuits to produce higher voltages, currents and power levels, to produce a large-scale solar power system. For example, groups of PV cells 100 may be electrically configured into prewired units called PV modules 110 with one or more PV modules 110 assembled as a pre-wired, field-installable unit, or PV panel 120. Finally, a PV array 130 is the complete power-generating unit, including any desired number of PV panels 120. PV systems may operate in parallel with, and interconnected to, the utility grid. For example, the Agua Caliente solar power system in Arizona has approximately 5 million PV panels and generates about 290 megawatts of power, enough electricity to power about 230,000 homes at peak capacity.
Like any other manufactured device that is exposed to environmental stresses, PV devices and their performance degrade with time and possess finite lifespans. Because it has no moving parts (the major source of reliability issues in other types of electrical generating systems), a PV device's operating life is largely determined by the stability and resistance to corrosion of the materials from which it is constructed.
Thus, as PV technology becomes more efficient and economical, continued growth and investment into the PV industry requires accurate predictions of PV device degradation and degradation rates. In addition, PV array owners and operators require methods for identifying the formation of defects in their PV devices operating in the field to assist with maintenance planning and scheduling to insure that their power plants continue to perform at the plants' nameplate power capacities.
To address these needs, various groups have developed inspection methods for assessing the condition and performance of PV devices. For example, some methods have utilized electroluminescence and/or photoluminescence imaging methods. For example, the Daylight Luminescence System (DaySys) developed at the Institute of Photovoltaics at Germany's University of Stuttgart (Daylight Luminescence for Photovoltaic System Testing L. Stoicescu, M. Reuter, and J. H. Werner in Proc. 22nd International Photovoltaic Science and Engineering Conference, edited by: (Hangzhou, China, 2012), (2012)). In this example, the PV device is connected to a modulation device, and an algorithm extracts electroluminescence generated images from a video stream of the PV device. However, this method requires electrical connection of the PV device being tested to an external power source, e.g. biased voltage supply. Photoluminescence, which typically involves illuminating and imaging the same section of a PV cell, causes photoluminescence of all of the PV cell being analyzed, regardless of whether portions of the section contain disconnects (e.g. due to cracks and/or breakage).
Thus, most methods for detecting defects in PV devices today are either limited to laboratory scale testing, require expensive specialized lasers and filters, or require connecting some sort of modulating and/or bias providing device. Therefore, although progress has been made in developing methods for detecting defects in PV cells, PV modules, PV panels, and/or PV arrays in the field, there is still significant need for simpler, safer, faster, more scaleable and more reliable methods and systems that detect defects in PV devices in the field and where ever PV arrays are used.